1. Field of the Invention
The present invention relates to data acquisition systems and methods for oil and gas wells. In one aspect, the present invention relates to systems and methodologies for determining temperature gradients in connection with thermal recovery projects involving oil or gas wells.
2. Description of the Related Art
There exists throughout the world major deposits of heavy oils which, until recently, have been substantially ignored as sources of petroleum since the oils contained therein were not recoverable using ordinary production techniques. These oil deposits are also referred to as heavy oil, or bitumen. For example, it was not until the 1980's that much interest was shown in the heavy oil deposits of the Alberta province in Canada even though many deposits are close to the surface and represent an estimated petroleum resource upwards of many billion barrels.
It is well known that heat can be employed to recover hydrocarbons from underground formations such as those found in the regions noted above. Often referred to as thermal recovery projects, the well owner uses heat to reduce the viscosity of the petroleum to a level where it will readily flow to wells from which it can be recovered to the surface of the earth. Steam and/or hot water flooding are commonly used for this purpose and have been very successful in some formations for stimulating recovery of viscous petroleum which is otherwise essentially unrecoverable. Steam flooding is a thermal oil recovery method which has enjoyed increased popularity in recent years and is often the most commercially practical method or process.
Steam flooding can be utilized in a single well by the so called “huff-and-puff” technique. That method involves first injecting steam into a vertical well, then shutting in the well for a “soak”, wherein the heat contained in the steam raises the temperature and lowers the viscosity of the petroleum. Thereafter, a production period begins wherein mobilized petroleum is produced from the well, usually by pumping. This process can be repeated over and over again.
Steam flooding may also be utilized as a thermal drive means by injecting steam into the reservoir through one or more vertical injection wells. This steam then moves through the subterranean reservoir mobilizing and volatilizing the petroleum it encounters. This steam-flood front moves through the reservoir towards a production well wherefrom the petroleum fluids are produced. This steam drive process is often more effective than the “huff-and-puff” method inasmuch as the potential volume of the reservoir which can be swept by the process is greater.
Alternate methods are also available to heat and mobilize the oil or bitumen in the reservoir. These include the use of hot solvent, hot gas, hot air as well as underground burning or combustion.
Thermal oil recovery projects such as those described above require temperature monitoring of the downhole temperature in the reservoir. Conventionally, temperature data is obtained via sensors positioned in both vertical observation wells and horizontal production wells. Historically, monitoring of thermal oil reservoirs utilized either multi-point thermocouple sensors or distributed temperature fiber optic cable in order to obtain multi-point readings and therefore infer a temperature profile in the reservoir. Many individual points are desired and therefore it is necessary to obtain a multitude of readings. Twenty points are often obtained in order to create a temperature profile across the formation. Typically, a thermocouple (e.g., a mineral insulated (MgO) thermocouple) is positioned at each selected temperature measurement point in the wellbore and connected to a surface interface with two wires. As is known, two wires are required for each thermocouple because two different alloys are connected together at the junction of the thermocouple. As can be appreciated, the costs of such wiring can become considerable because the length traversed by the wires can approach several kilometers. Furthermore, the data transmitted across these relatively long spans of wiring or cables can degrade and weaken and be corrupted by noise. The long lengths are also prone to accuracy loss caused by wire impurities, insulation breakdown, and low voltage signal degradation. Likewise, fiber optic cables sensors as well as the associated fiber optic cables are relatively expensive and prone to degradation in signal accuracy and strength due to hydrogen related problems with the glass fiber. Moreover, in both instances, the splices or couplings between individual lengths of cable can introduce noise and errors into the transmitted data. Yet another difficulty and expense arises from the need to effectively seal and pack-off multiple wires or multiple bundles of cables exiting the wellhead. As is known, effective wellhead sealing can be essential to containing a well blow out.
The present invention addresses these and other drawbacks of the prior art.